As the computer technology has recently been developed, a technical innovation is greatly introduced in printing. The field of color printing is now digitized by using a computer system named as CEPS (Color Electronic Prepress System). The CEPS digitizes the printing data of a color image and the data of all the prepress process down to stripping. Further, the newest system directly outputs the digitized printing data to a presensitized printing plate of a high sensitivity to prepare a printing plate without use of a conventional lith film. The technical innovation in the printing field makes the lith film unnecessary.
Before running on, a proofread is necessary to inspect the finished quality of printing. The proofing systems include a pressproof and an analogue offpress proof. The pressproof comprises preparing a printing plate for a proof sheet and printing an image using a proof press. The analogue offpress proof employs an image forming system different from the printing system. The offpress proof system comprises uniformly exposing to light a light-sensitive material through a lith film. The light-sensitive material contains a photopolymer, and the light source usually is ultraviolet. These two proofing systems are based on the premise that the printing system uses a lith film. The proof sheet itself is prepared from the image on the lith film (cf., M. H. Bruno, Principle of Color Proofing).
As is mentioned above, the conventional proofing systems are based on use of the lith film. Now, it is an important problem how to inspect the printing quality when the technical innovation in the printing field makes the lith film unnecessary. Therefore, the newest printing system requires a proof system of preparing a proof sheet directly from digital image data without use of the lith film. The required proof system is named as DDCP (Digital Direct Color Proof). With respect to the DDCP, several systems have been proposed, and some of them are practically used.
The DDCP systems employ an electrophotography and a thermal transfer recording system, which are completely different from the conventional analogue offpress proof using a photopolymer. The proofing qualities of DDCP (such as resolving power, color tone, gradation, reproducibility of half tone, analogy to printing and stable reproducibility of the image) are insufficient compared with the conventional systems. The image qualities of the direct output from a digital image are not satisfied. Therefore, an improved proofing method which can directly output the digital image is required to obtain a color proof of high quality.
Japanese Patent Publication No. 48(1973)-31323, Japanese Patent Provisional Publications No. 59(1984)-97140, No. 62(1987)-267736, No. 2(1990)-244151 and U.S. Pat. No. 3,770,438 disclose an analogue offpress proof using a conventional photopolymer. If the digital image data can directly be recorded on the offpress prof using the photopolymer, the proofing qualities are probably satisfactory. However, the digital image data should be recorded on a light-sensitive material for proof by a scanning exposure using a specific light source such as a laser bean. Therefore, the light-sensitive material should have a high sensitivity to the wave length of the scanning light. The conventional light-sensitive material using a photopolymer does not have such a high sensitivity. Further, it is particularly difficult to sensitize the photopolymer to a laser beam of a long wave such as a He-Ne laser and a semiconductor laser. The photopolymer merely has a sensitivity to a laser beam of a short wave. Accordingly, the conventional analogue offpress proof is not available in the DDCP systems.
In conclusion, the offpress proof using a photopolymer cannot directly record the digital image data, since the photopolymer does not have a high sensitivity to the wave length of the scanning light (particularly a laser beam of a long wave). On the other hand, the proofing qualities of the conventional DDCP systems (using an electrophotography or a thermal transfer recording system) are insufficient with respect to resolving power, reproducibility of half tone or stable reproducibility of color. Therefore, a new DDCP method is required to record a color proof of high quality with a high sensitivity to the wave length of a scanning light.
By the way, Japanese Patent Provisional Publications No. 61(1986)-69062 and No. 61(1986)-73145 (the contents of these two publications are described in U.S. Pat. No. 4,629,676 and European Patent Provisional Publication No. 0174634A) discloses an image forming method using a light-sensitive material which contains silver halide, a reducing agent and a polymerizable compound. The method comprises the steps of imagewise exposing to light the light-sensitive material, and heating the material to develop the silver and to harden imagewise the polymerizable compound. Thus a polymer image is formed on the light-sensitive material. In a representative embodiment of the method, the developed light-sensitive material (containing a colorant) is pressed on an image receiving material to transfer the colorant with the unpolymerized polymerizable compound to the image receiving material. Thus a color image is formed on the image receiving material. In another embodiment, the unhardened area of the polymerizable compound containing the colorant is removed with a solvent to form a color image on the light-sensitive material.